Recently electrochemical biosensors are frequently used in medical field to analyze biomaterials including blood. Among those, enzyme-utilizing electrochemical biosensors are used most generally in hospitals and clinical labs because they are easy to apply, superior in measurement sensitivity, and allow rapid acquisition of test results. Enzyme analyzing method applied in electrochemical biosensors can be largely divided into chromophoric method which is a spectroscopic way and electrode method, an electrochemical way. Generally, the measuring time in chromophoric method takes longer than electrode method, and difficult to analyze significant biomaterials due to the measurement errors caused by the turbidity of biomaterials. Therefore, an electrode method is extensively applied in electrochemical biosensors recently. According to the method, in an electrode system established by screen printing, the quantitative measurement of a material of interest can be achieved by fixing a reagent onto the electrodes, introducing a sample, and applying an electric potential across the electrodes.
U.S. Pat. No. 5,437,999, “Electrochemical Sensor”, discloses an electrochemical biosensor test strip with a precisely defined electrode field applying technologies generally used in PCB industries adequately to an electrochemical biosensor test strip. This electrochemical biosensor test strip can operate analysis very precisely with a small amount of samples.
FIG. 1 is a plan view of a conventional electrochemical biosensor test strip. In FIG. 1, 11 is a recognition electrode, 12 a reference electrode, 13 a working electrode and 14 a reaction portion on which a reagent is fixed.
FIG. 2 is a circuit diagram of a conventional electrochemical biosensor readout meter using the test strip 10 shown in FIG. 1, FIG. 3A is a waveform of the working voltage applied to the working electrode 13 by the working voltage generating circuit 21, and FIG. 3B is a waveform of the electric current flowing in the working electrode 13 depending on the introduction of sample.
Below, referring to FIG. 2 and FIG. 3, the operation of a conventional electrochemical biosensor readout meter 20 will be described. When a test strip 10 as shown in FIG. 1 is inserted into the readout meter 20, the voltage of point A changes into 0V from 5V. This change of voltage is recognized by a microprocessor 26 serving as a controller, and the insertion of the test strip can be detected. At this point of detecting the insertion of the test strip (t0), the microprocessor 26 controls a working voltage generating circuit 21 to apply a fixed voltage, for example 300 mV, to a working electrode 13.
When blood and the like is introduced to the reaction part 14(t1), a material to be analyzed from blood reacts with a reagent, generating electric charges. And these electric charges form the electric current by the voltage which has been applied to the working electrode 13. The electric current increases depending on the advance of reaction between the reagent and the material to be analyzed as shown in FIG. 3B. When the current becomes a certain amount(ith)(t2), the microprocessor 26 controls the working voltage generating circuit 21 not to apply any voltage to the working electrode 13. The reason for waiting until the current becomes a certain amount(ith), is to prevent malfunctioning by noise etc.
Since the working voltage is substantially 0V, the electric charges generated by the reaction between the material to be analyzed and the reagent, cannot flow via the working electrode 13, gathering around the working electrode 13. After the working voltage is substantially 0V, at the point t3, the working voltage of 300 mv is applied to the working electrode 13. Here, the time from t2 to t3 is generally called ‘incubation time’. The electric charges gathering around the working electrode during incubation time, simultaneously come to flow via the working electrode 13, when the working voltage of 300 mv is applied to the working electrode at t3. Therefore, as shown in FIG. 3B the peak electric current(Ip) emerges at t3.
Referring to the circuit diagram in FIG. 2, the principle of measuring the concentration of a material to be analyzed by measuring the current flowing in the working electrode 13 is described as follows. The current flowing in the working electrode 13 is converted into the voltage by the resistance (R1) which is in feedback-loop of the output terminal and the (−)input terminal of the operational amplifier. This converted voltage is changed into a digital signal by the analogue-digital(A/D) converter 23. The microprocessor 26 has in store the data on the relations of the material to be analyzed from sample to the current. The microprocessor 26 measures the concentration of the material to be analyzed, by reading the current flowing in the working electrode 13 at the time of t4 at which the peak current(Ip) has passed to some degree. The reason for measuring the concentration of the material to be analyzed at t4, is that the value of peak current varies with the state of coupling the reagent to the reference electrode and the working electrode, although the concentration of the material to be analyzed from sample is same.
As described above, so far there was no voltage applied to the working electrode during the incubation time, so that the peak current at t3 was very high. Therefore, if the resistance R1 becomes high, the distortion of signal appears nearby t3 at which the peak current occurs corresponding to the limitation of the operational amplifier OP1, accordingly the current at t4 is also affected. FIG. 4a is the current waveform in case that the resistance R1 is so small that the current flowing in the working electrode can sufficiently flow nearby t3. And FIG. 4b is the current waveform in case that the resistance R1 is so large that the current flowing in the working electrode cannot sufficiently flow nearby t3. In this case, the value of peak current varies with the state of coupling the reagent to the reference electrode and the working electrode so that the current measured at t4 varies with the test strip used. Accordingly, there was the problem of reproduction. Also, if the resistance R1 is decreased so as to let a large peak current flow without distortion, the waste of expenses is occurred since the voltage measured at t4 is relatively much smaller than the voltage at t3 and every bit of A/D converter 23 cannot be used.
Besides, a conventional biosensor readout meter used only one operational amplifier OP1 so as to convert the current flowing in electrodes into the voltage, as shown in FIG. 2. For example, when the reference voltage of the A/D is 3.7V, the value of the resistance R1 100 kΩ and the (+)power supply voltage of the operational amplifier 5V, the current range measurable at t4 is 0<i<37 μA and the maximum value of peak current allowable in the operational amplifier is 50 μA. If the value of peak current is to be raised, the maximum current range measurable at the time of t4 becomes higher than 37 μA. In case the conversion bit of the A/D converter 23 is 8 bit, if the maximum range of current grows larger, the resolution grows worse. Therefore, to gain the preferable resolution the conversion bit should be raised. In such a case, since an expensive A/D converter should be used there was the problem of a rise in expenses.